The invention relates in general to the field of materials processing, and more particularly to processing materials for manufacturing substrates for electronics, optics, or optoelectronics, or indeed for manufacturing Microsystems. In particular, the invention relates to a method and to apparatus for performing separation operations on planes of weakness in such substrates, and for doing so in a manner that is controlled and precise.
More specifically, the invention relates to a method and to apparatus for separating on a separation plane two wafers initially placed one against the other with varying degrees of mechanical cohesion.
A known method of manufacture, silicon on insulator (SOI), comprises a step of implanting ions to given depth in a monocrystalline silicon wafer, a step of fixing said wafer on a stiffener such as silicon that has optionally been oxidized on the surface, and then a step for ensuring that cleavage takes place at least in part on a plane of weakness as defined by the layer of implanted ions. (In some cases, if the thicknesses of the two portions of the monocrystalline silicon wafer situated on either side of the plane of weakness are sufficiently great so that they themselves present enough mechanical strength, the step of fixing on a stiffener can be omitted.)
In that type of method, whether cleavage takes place completely or occurs in part only, the two wafers remain stuck together in practice (merely by a suction cup effect even when cleavage is complete), and they still need to be separated so as to obtain firstly the final SOI substrate which will subsequently be subjected to various finishing treatments, and secondly the remaining monocrystalline silicon which can be recycled in the method.
Such separation must naturally be performed with the greatest care in order to ensure that the two wafers come apart, where necessary while also finishing off cleavage, without running the risk of damaging the two wafers.
In general, this operation is performed manually by a particularly skilled operator, for example by inserting a sharp blade or the like into the edge of the plate level with the plane of separation so that separation can be achieved by a wedging effect. This operation runs the risk of causing the facing faces of the two wafers to be subjected to shocks and to friction, thereby damaging them. In addition, this manual operation is lengthy and fiddly, and production throughputs are greatly constrained thereby. Finally, particularly when cleavage between the two wafers is to be terminated by the separation process itself, the forces applied to the plate need to be particularly great and the above-mentioned manual operation becomes inappropriate, or even dangerous.
The present invention thus seeks to provide a method and apparatus for enabling such separation to be performed quickly, reliably, and reproducibly, and which also avoids any contact or friction between the wafers while they are being separated, and thus any risk of scratching or of particles being deposited on the active faces of said wafers.
Another object of the invention is to be able to separate wafers which cohere mechanically to an extent that can vary very largely, in particular wafers between which macroscopic cleavage is partial only, and without it ever being necessary to exert excessive forces on the plate.
Thus, in a first aspect, the present invention provides a method of separating into two wafers a plate of material for manufacturing substrates for electronics, optics, or optoelectronics, or for manufacturing microsystems, said wafers being situated on either side of a plane of weakness, the method being characterized in that it comprises the steps consisting in:
exerting a deformation force on at least one of the wafers so as to cause the wafers to separate from each other in a zone of the plate at said plane of weakness; and
exerting guided separation movement on the wafers.
By means of the invention, by exerting a deforming force it is possible in a localized zone of the plate to initiate a separation wave in the plane of weakness. This wave can propagate over the entire extent of the plane of weakness either as soon as the deformation is applied thereto, or else once the separation movement of the two wafers has begun, but in any event extremely quickly and without any need to apply large separation forces.
If forces were to be exerted from opposite sides of the wafers over the entire area without having a localized start of separation, then it would be necessary to integrate the force required for achieving separation over the entire surface area, and that would amount to a considerable force.
However, in the method of the invention, the force to be applied to achieve separation proper is very limited, making it possible to achieve this separation on a path that is extremely well controlled, without any risk of shock or friction between the surfaces being separated, which is particularly important when the surfaces are to become active surfaces for which quality requirements in terms of purity, shape, etc. are particularly critical.
Advantageously, the separation start is achieved by exerting a main stress in the vicinity of an edge (or of the single edge when the plate is in the form of a disk), on at least one of the faces of the plate but not on the edge surface thereof, even though certain variants of the invention allow for the combination of stresses being applied both on the faces and on the edge surface.
Thus, in accordance with another object, the invention makes it possible to separate the wafers on a single plane of weakness that is well determined.
In the specification, the term xe2x80x9cplane of weaknessxe2x80x9d means a zone of material obtained after the material has been subjected to special treatment, which zone extends over a certain thickness perpendicularly to said plane. Such a treatment can comprise implanting an atomic species, making the material porous, etc. In any event, the treatment is such as to enable the structure of the material in said zone to be modified or as to ensure that the material has a special structure in said zone so that separation by the method and/or the apparatus of the invention takes place preferentially in said zone. The plane need not be a plane of the crystal lattice in the strict meaning of that term, and separation can take place in and can extend perpendicularly to said plane so as to occupy a plurality of crystal lattice planes, depending on irregularities in the materials.
Preferred features of the method of the invention are as follows:
the step of applying a deformation force comprises applying suction on at least one of the wafers in a region which extends locally close to the vicinity of an edge of the plate;
the step of applying a deformation force comprises applying suction on at least one of the wafers in a region which extends from close to the vicinity of the edge of the plate to the center thereof; advantageously, under such circumstances, the suction is applied by gripping means of stiffness greater than that of the wafers to be separated;
advantageously, the deformation is produced by applying a force on a zone of the plate which is situated between at least two thrust points that are rigidly secured to each other; advantageously the amplitude of the deformation in the direction perpendicular to the main surfaces of the plate is less than 1 mm, and preferably less than 500 xcexcm; this characteristic makes it possible to control and to limit deformation of the wafers and the faults that could result therefrom;
the step of applying a deformation force is advantageously suitable for giving rise to shear stress in at least one region of the plane of weakness; advantageously, this deformation is implemented by curving the plate in a direction perpendicular to its surface;
the separation step comprises moving means suitable for applying said suction in a direction that extends generally transversely to the plane of the plate;
a the separation step consists in moving at least one of the wafers in translation without rotation;
the separation step consists in moving at least one of the wafers in translation accompanied by rotation;
the method further includes a step of releasing at least one of the wafers by eliminating the suction;
the method further includes a step of applying stress suitable for favoring separation; and
said application of stress is taken from the group comprising: applying a temperature gradient, applying mechanical vibration, applying chemical etching, applying shear stress, and applying a flow of fluid at high speed.
If the stress suitable for favoring separation is shear stress, it can be obtained by heat treatment when the materials on either side of the plane of weakness do not have the same coefficient of thermal expansion or when these materials are not at the same temperature. It can also be obtained by deforming the plate, e.g. by deformation tending to give the plate a convex shape around an elongate cap.
The stress suitable for favoring separation can correspond to a combination of the above-specified stresses, either simultaneously or separated in time.
The deformation force and the stress suitable for favoring separation can both be of the same kind and can correspond to a single operation.
In a second aspect, the invention provides apparatus for separating a plate of material, in particular of semiconductor material, into two wafers, said wafers being situated on either side of a plane of weakness, the apparatus being characterized in that it comprises:
means for holding the plate via a first wafer;
stress-applying means suitable for exerting plate deformation stress at least on the second wafer; and
means for moving the wafers apart from each other along a predetermined path.
Preferred but non-limiting features of the apparatus of the invention are as follows:
the stress-applying means and the separation means are constituted by the same gripping member;
said gripping member possesses suction means;
the gripping member includes a generally plane element whose face directed towards said wafer possesses at least one suction cavity;
a single suction cavity is provided that is generally elongate in shape and that is suitable for extending between an edge region of the plate and a central region of the plate;
the holding means is constituted by a second gripping member extending substantially parallel to the first gripping member;
the device further includes guide means on which the first gripping member is mounted;
the device further includes means for resiliently urging the first gripping member away from the second gripping member;
the device further includes manual actuator means suitable for urging the first gripping member against said resilient urging so as to move the two gripping members towards each other;
the guide means and the resilient urging means are constituted by the same member;
said member is a single-piece link member constituting a deformable parallelogram; and
it further includes means for selectively eliminating the suction.
Other features, objects, and advantages of the invention and apparatus of the invention appear more clearly on reading the following detailed description given with reference to the accompanying drawings, in which:
FIG. 1 is a perspective view of a first embodiment of separation apparatus of the invention;
FIG. 2a is an overall elevation view of a gripping portion of the FIG. 1 apparatus and of a plate to be separated into two wafers;
FIG. 2b is a cross-section view through the assembly of FIG. 2a;
FIG. 2c is a perspective view of the assembly of FIGS. 2a and 2b;
FIG. 3a is a perspective view of a part forming a deformable parallelogram suitable for fitting to the apparatus of FIG. 1;
FIG. 3b is a diagrammatic representation of the deformable parallelogram in a first position;
FIG. 3c is a diagrammatic representation of the deformable parallelogram in a second position;
FIG. 4a is a fragmentary section view on a mid-longitudinal plane showing two gripping members situated on either side of a plate when initiating separation;
FIG. 4b is a fragmentary section view on a plane extending transversely to the gripping members, under the same circumstances;
FIG. 5 is a diagram showing how a separation wave propagates in the plate; and
FIG. 6 is a diagrammatic perspective view of a second embodiment of apparatus of the invention.